housekeeping: Update README.md

[skip ci]

README.md

@@ -31,28 +31,40 @@ Stun.Net | [![NuGet.org](https://img.shields.io/nuget/v/Stun.Net.svg?logo=nuget)
 ![](pic/1.png)
 
 ## RFC3489
+<details>
+
 ![](pic/RFC3489.png)
+</details>
 
 ## RFC5389
 ### Binding Test
-Checking for UDP Connectivity with the STUN Server
+<details>
+  <summary>Checking for UDP Connectivity with the STUN Server</summary>
 
 ![](pic/RFC5780_4.2.png)
+</details>
 
 ### Mapping Behavior
-Determining NAT Mapping Behavior
+<details>
+  <summary>Determining NAT Mapping Behavior</summary>
 
 ![](pic/RFC5780_4.3.png)
+</details>
 
 ### Filtering Behavior
-Determining NAT Filtering Behavior
+<details>
+  <summary>Determining NAT Filtering Behavior</summary>
 
 ![](pic/RFC5780_4.4.png)
+</details>
 
 ### Combining Tests
+<details>
 
 ![](pic/RFC5780_4.5.png)
 
+</details>
+
 ## STUN Server
 ### Docker
 ```